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13D and 13G filings with the SEC allow observers and individual investors to track large positions being taken by hedge funds, including some of the best known and most successful investors today. Here are what five hedge funds have been up to recently:

Citadel Advisors has been working overtime on its 13G filings. Billionaire Ken Griffin’s fund announced a 5% stake in Marriott Vacations Worldwide (NYSE:VAC). Marriott Vacations is a spin-off of the larger Marriott company, and is the largest pure-play time share management company in the world. Marriott’s business has been stagnant recently, with small gains in revenue in North America being offset by declines in the rest of the world. The stock is up 13% this week, so Griffin had likely made some good gains on his investment so far. And just last week Citadel had announced a 15.1- million share position in Bakken Shale-focused oil exploration and production company Kodiak Oil and Gas (NYSE:KOG). Read more about Ken Griffin’s investment in Kodiak. With one big buy in an oil-related business and another in a vacation-oriented business, we think this signals that Griffin is bullish on the U.S. and global economies even as many investors worry about low growth.

Greenlight Capital had told us this one was coming: David Einhorn has reported a 60% increase in his position in Marvell Technology Group (NASDAQ:MRVL). Einhorn said in his letter to investors at the end of the second quarter that “MRVL has about $4 per share in cash and now trades at roughly5x next year’s earnings…the company has commenced what we hope will be an aggressive share repurchase program. We have used the reduced stock price as an opportunity to increase our stake in the company.” See what else Einhorn reported to Greenlight Capital’s investors. Greenlight now owns nearly 30 million shares of Marvell, which counts processors, data storage, and communication products among its offerings.

Steve Cohen’s SAC Capital Advisors(see more of Steve Cohen’s stock picks) bought into electric utility GenOn Energy (NYSE:GEN) and now owns approximately 36.7 million shares of the company, which comes close to 5% of the shares outstanding. GenOn is currently in the process of merging with larger utility NRG Energy (NYSE:NRG) and its shareholders will receive a premium of about 20% of the stock’s price prior to the merger announcement. Cohen may be engaging in traditional merger arbitrage because he believes he can exploit the difference between the current stock price and what shareholders would receive in the case of a merger (learn more about merger arbitrage strategies) or he may believe that NRG will be forced to sweeten the deal. Read further coverage of Cohen’s move.

Third Point Management, whose manager Dan Loeb has been a tireless activist investor in Yahoo (NASDAQ:YHOO) and played a key role in driving out its former CEO, gave new leader Marissa Mayer its seal of approval. Third Point added 2.5 million shares shortly after Mayer took over and owns about 6% of the shares outstanding, giving the fund a major upside if Mayer is able to turn the company around and increase earnings. However, Yahoo faces a legion of higher status mega-cap technology companies in any direction that it might attempt to expand.

Finally, Fairfax Financial Holdings, which had previously owned about 5% of Research in Motion (NASDAQ:RIMM), decided that the plunge in the share price as BlackBerry ownership declines- the stock is down about 50% so far in 2012- had given it a good entry point. Fairfax now reports owning about 10% of the shares outstanding, a total of nearly 52 million shares. Fairfax is run by Prem Watsa, whose value investing skills have earned him the nickname “the Warren Buffett of Canada.” Find more information on Fairfax and RIMM.